This invention relates to a high-efficiency audio amplifier circuit with switching of the operating state depending on the input signal.
The so-called bridge configuration is usually used for audio amplification in radio sets for motor vehicles and requires two power amplifiers driving the load with two signals in opposite phase; the amplifiers are normally of class AB. With this configuration the high powers required by the market can be obtained, typically 20 W on a 4-ohm load, with a 14.4 V power supply.
The efficiency curve of this type of amplifier has a logarithmic shape, starting from 0% at null power and reaching 70% at maximum output power. Typical power and efficiency curves for this configuration are illustrated in FIG. 1, which is a plot of the dissipated power P.sub.d and of the efficiency against the output power P.sub.o. Since the average value of the input signal is 20-30% of the maximum deliverable power (4.6 W for a maximum power of 20 W), as shown by statistical research on the behavior of the musical signal which normally drives these amplifiers, it follows that the average efficiency of the amplifier is in the range 0% to 30%.
FIG. 2, on the other hand, is a similar plot concerning one single-ended amplifier driving the load via a series capacitor, with power supply and load as above.
In order to obtain higher efficiencies and therefore lower dissipation for the same power delivered, it is known to use class D switch amplifiers instead of said class AB amplifiers, or to use devices which modulate the power supply of class AB amplifiers depending on the amplitude of the output signal. Both these prior approaches, however, have severe disadvantages, such as a high degree of complexity or undesirable radiation effects, and furthermore require special power supplies.